The classic technique for determining moisture content in fluids requires sample handling in the mode described in U.S. Pat. No. 2,830,945, to Keidel, which is considered exemplary of the technique. Here the sample is taken and passed through a flow meter to determine volume. Essentially all water is then extracted from the sample onto an absorbent film. Substantially complete hydrolysis is practiced to reduce the extracted water to its components O.sub.2 and H.sub.2, and the expended current is related to the sample volume to quantitate the H.sub.2 O concentration.
Obvious disadvantages of the technique include fouling problems, since total electrolysis is practiced. While sample filtering is useful to reduce the scope of the problem, and is in fact, required for most applications, the step itself introduces even further complexity into the technique, and is usually not entirely successful. More objectionable is the slow response required since the technique is based on sampling rather than a direct analysis. What the art refers to as "combination effects" or non-selective reactions are also produced in the hygrometer cell, thus producing a response interference that lessens precision. Also, the technique is limited further in application by the percentage of H.sub.2 O concentration. Hence, for an analysis above 10,000 ppm, the technique is oftentimes not suitable.
The only known prior direct measurement technique, thus avoiding some of the problems with the earlier Keidel mode, is based on a novel probe construction using an aluminum base, a porous aluminum oxide layer thereon, and a thin gold coating deposited on the oxide layer. The device is available from Panametrics, a subsidiary of Esterline Corporation. The principle relies on the selective transportation of water molcules through the thin gold plating and into the porous aluminum oxide coating. Impedance changes in the probe thus produce a value from which H.sub.2 O concentration can be deduced. The device, however, is not useful for a diverse number of industrial chemical and other process stream analysis simply because it is not chemically durable. Also, while the principle might be possibly expanded to use other membrane constructions, such as the aliphatic fluorocarbon membranes mentioned in the abstract (which have been suggested for the Keidel electrolysis technique), actual experiments (not considered prior art) have failed to establish suitability of purpose because of material expansion problems.